1. Field of the Invention
The present invention relates to a technique for realizing genome shuffling by efficiently repeating outcrossing of plants that are usually self-pollinating such as rice and wheat (i.e., autogamous plants) without involving laborious procedures such as emasculation and identification of male-sterile individuals; and to an efficient recurrent selection breeding system based on the technique.
2. Description of the Related Art
Breeding of allogamous plants such as corn originally involves recurrent selection. Specifically, genome shuffling is caused due to frequent genetic recombination between homologous chromosomes of heterozygotes and increases genetic variability, enhancing breeding effects.
Meanwhile, for the production of cultivars in autogamous plants such as rice and wheat, selfing is generally repeated from the juvenile screening stage and thus fixed strains are selected. Therefore, frequent genetic recombination can be observed in only a few generations, which perhaps impose limitation on the breeding of autogamous plants.
Primarily, the breeding effects depend greatly on “increased variability through genetic recombination between numerous alleles” and “applying appropriate, continuous selection pressure to a population.” Thus, an ideal breeding system is a recurrent selection breeding system that can perform efficient genome shuffling and also apply strong selection pressure to a large population.
Nuclear male sterility is effectively utilized for realizing recurrent selection based on genome shuffling which is achieved by efficiently outcrossing autogamous plants. As a method for realizing such recurrent selection, the MSFRS (Male Sterile Facilitated Recurrent Selection) method is proposed (see Ramage, R. T. (1975) Techniques for producing hybrid barley. Barley Newsl. 18: 62-65; and Eslick, R. F. (1977) Male sterile facilitated recurrent selection-advantages and disadvantages. Proc. 4th Regional Winter Cereals Workshop (Barley). Vol. II. 84-91). The MSFRS method aims to realize recurrent selection based on efficient genome shuffling and to obtain high breeding effects. Specifically, the MSFRS method includes the following steps: 1) selecting male-sterile individuals or male-fertile individuals from a population containing both the male-sterile and male-fertile individuals and producing F1 population by crossing together the selected male-sterile individuals and the selected male-fertile individuals, 2) producing a population of F2 individuals for the next selection cycle, 3) introducing new genetic resources into a population in each cycle through outcrossing with male-sterile individuals, and 4) repeating the selection cycle.
However, the MSFRS method requires discrimination between male-sterile individuals and male-fertile individuals during the flowering period. Thus, the MSFRS method is difficult to use when recurrent selection is efficiently performed in large populations. To resolve this problem, it has been proposed to use, as a marker trait, a seed trait linked with male sterility, for example. However, this method cannot be a universal method since the male-sterile gene must be linked closely with the marker gene. In addition, this method possesses a problem in that the linkage between the marker gene and the male-sterile gene are sometimes broken as a result of genetic recombination therebetween.
Furthermore, other literatures have reported a method in which dominant male-sterile individuals are produced utilizing an anther-specific promoter and a lethal gene (e.g., a ribonuclease-encoding gene) by the transgenic technique (see, for example, U.S. Pat. No. 6,509,516; Mariani, C., M. De Beuckeleer, J. Truettver, J. Leemans, and R. B. Goldberg (1990) Induction of male sterility in plants by a chimaeric endonuclease gene. Nature. 347: 737-741; and Mariani, C., V. Gossele, M. De Beuckeleer, M. De Block, R. B. Goldberg, W. De Greef, and J. Leemans. 1992. A chimaeric ribonuclease-inhibitor gene restores fertility to male sterile plants. Nature (London) 357: 384-387). Also, by introducing a chemical tolerance marker gene (e.g., an herbicide-tolerance marker gene) as the same construct, dominant male-sterile individuals can be selected at the seedling stage. The thus-produced transformants have dominant male sterility and herbicide tolerance in a tight coupling linkage. This method is used for producing F1 seeds of Brassica napus L. in North America.